1. Field of the Invention
The present invention relates to a color image recording method utilizing an electrophotographic recording method and particularly to an improvement in the color image recording method in such a type as recording color images by forming a color image of a plurality of colors on a latent image carrier such as a photosensitive material, etc. and transferring such color image of a plurality of colors simultaneously to an image acceptor such as a recording sheet.
2. Description of the Prior Art
Various kinds of methods have been proposed as color image recording methods utilizing an electrophotographic recording method and so-called superimposed development methods. In this superimposed development method, a plurality of color images are formed on a latent image carrier by sequentially superimposing and developing images in a plurality of colors on the relevant latent image carrier, such as a photosensitive material drum, etc., and a color image can be obtained by transferring at a time such a plurality of color images on the recording sheet.
This method has an advantage that an image can be formed at a higher rate because the apparatus can realize size reduction since it requires only one latent image carrier such as a photosensitive material drum and does not require a drum for holding a recording sheet and a transfer drum and because a plurality of color images can be formed while the latent image carrier makes a turn by providing a plurality set of a latent image forming device and a developer unit around the latent image carrier.
In such a superimposed development method, it has been an extremely important technical problem how one may skillfully execute the second and subsequent development processes without disturbing or removing a toner image on the latent image carrier because the already developed toner image passes again through the development area in the development processes for second and subsequent colors following the development process for the first color.
As a method of developing an electrostatic latent image formed on a latent image carrier, various methods employing a so-called contact type double-element magnetic brush development method have been proposed. In these methods, for example, an electrostatic latent image is visualized by placing the surface of the latent image carrier in contact with the double-element developer consisting of toner and magnetic carrier. This development method is thought of as a typical development method because it has some problems that toner density control is necessary and an apparatus becomes large in size but it is superior in image quality, sustainability and conveyability of the developer.
When the contact type double-element magnetic brush development explained above is employed in the second and subsequent development processes of the above-mentioned development method, a toner image of the first color generates brush-mark or sweeping by magnetic brush, resulting in the disadvantage that a toner image may be disturbed easily.
Moreover, a toner image of the first color is removed from the latent image carrier and is then mixed into the second developer in the second development process, resulting in another disadvantage that the image density of the first color image is lowered and life expectation of the second developer is remarkably lowered.
Therefore, in view of overcoming such disadvantages, various techniques utilizing a so-called non-contact development method have been proposed for realizing development without allowing contact between the latent image carrier and the developer in the second and subsequent development processes.
As with the non-contact development method, a development method utilizing a vibration voltage consisting of an AC voltage to which a DC voltage is superimposed and a development method utilizing only DC voltage have been well known.
The latter method is inferior in the fine line reproducibility to the contact type development method because the development field working on the toner is rather weak. Moreover, a gap between a latent image carrier and a development roll must be set narrow to obtain sufficient developing electric field intensity, requiring a higher mechanical accuracy. Meanwhile, since the former method is superior in the developing field intensity working on the toner to the latter method and thereby the technical problem explained above can be improved, it is concluded that the former method has advantages to the latter method.
However, in the former development method, a novel technical problem is generated due to application of a vibration voltage.
Namely, the vibration voltage causes an intensive electric field to work on the toner to scatter it on the latent image carrier, resulting in a disadvantage of mixing of colors, where the toner of the subsequent stages is deposited to the toner image of the preceding stages on the latent image carrier.
Meanwhile, when an amplitude of vibration voltage is set higher in order to obtain sufficient image density, an electric field which works on the toner of the first color on the latent image carrier to reversely scatter it in the side of the development roll is intensified, resulting in a disadvantage that the toner image of the first color is electrically disturbed and removed.
Moreover, when the magnetic carrier electrostatically vibrates and a developer layer is placed in contact with a toner image of the first color on the latent image carrier, there is provided a disadvantage that the toner image is disturbed and removed as in the case of the contact development method.
In addition, there is also provided a disadvantage that the magnetic carrier is easily transferred onto the latent image carrier, that is, the carrier is deposited easily. If carrier is deposited, the carrier transferred on the latent image carrier is placed in contact with a recording sheet together with a toner image in the transfer region. Thereby, image quality is deteriorated, for example, missing or removal of toner image is generated and the carrier is transferred on the recording sheet to create a black point.
For this reason, various techniques for setting the developing bias voltage have been proposed to overcome such technical problems.
For example, an image forming method disclosed in the official gazette of the Japanese Patent Publication No. HEI 3-2304 is constituted to satisfy the formulae, EQU 0.2.ltoreq.VAC/(d.multidot.f) EQU {(VAC/d)-1500}/f.ltoreq.1.0
when an amplitude of the AC element of the developing bias is defined as VAC(V), frequency as f (Hz) and a gap between the latent image carrier and a developer carrier for transferring the developer as d (mm) in the development process of the second and subsequent colors.
Moreover, a multicolor electrostatic recording apparatus disclosed in the official gazette of the Japanese Patent Laid-Open No. HEI 2-77767 is not always based on the fact that it is employed for a double-element development system or non-contact type development system, but, in this multicolor electrostatic recording apparatus, the developing bias to be impressed to the development roll is set so that the waveform of such bias provides difference between a half value (1/2) and an average voltage value of the maximum voltage in one period of such waveform of bias voltage in the development process of the second color.
In addition, the above official gazette also discloses that the developing bias waveform is set as explained above, the maximum electric field working in such a direction as attracting a toner image of the preceding stage on the latent image carrier to the development roll is set to 2.3 V/m or less and the maximum electric field working in such a direction as scattering the developer on the development roll onto the latent image carrier is set to 2.8 V/m or more.
Furthermore, the image forming apparatus disclosed in the Japanese Patent Laid-Open No. HEI 3-206473 is constituted to adjust a duty ratio and a peak value of the developing bias voltage in the developer unit of the first color and the developer unit of the second color for each developer unit. Meanwhile, this official gazette also discloses that the developing magnetic poles are arranged anywhere desired other than the position where the development roll and latent image carrier are provided closest with each other and the double-element developer consisting of toner and magnetic carrier is held on the non-contact basis from the latent image carrier for the purpose of development.
However, the image forming method disclosed in the official gazette of the Japanese Patent Publication No. HEI 3-2304 still has such a technical problem that so-called carrier deposition resulting in deposition of carrier particles at the peripheral areas and areas between lines of line images is generated easily when an image having a higher space frequency such as a Chinese character having a large number of strokes is developed.
Carrier deposition is generated in such a manner that since an electrostatic latent image, where an image section and a background section are adjacently located keeping a very small clearance therebetween, exists at the surface of the latent image carrier in the case of a Chinese character having a large number of strokes having a higher space frequency, fringe field is generated at the boundary (edge) of the image section and the background section due to the electrostatic latent image at the surface of the latent image carrier. The carrier charged inversely from the toner is deposited to the edge portion and to the areas between lines of the image due to the fringe field having a higher electric field intensity formed at the edge portion of the image section.
Meanwhile, when the processing speed becomes higher and thereby the development roll rotates at a higher speed, a centrifugal force working on the carrier also increases bringing about a result that carrier deposition and scattering of carrier are generated more easily. This image forming method has a technical problem that it is not suitable for the high speed processing.
In addition, the image forming method of the type explained above has a technical problem that when the amount of charge of the developer is changed due to environmental change or passage of time, for example, when the amount of charge of the developer is increased under the low temperature environment or when it is increased with passage of time, electrostatic vibration of the magnetic carrier with impression of the vibration voltage increases with an increase in the amount of charge of carrier and thereby the developer layer is easily placed in contact with the surface of the latent image carrier. When the developer layer is placed in contact with the surface of latent image carrier, disturbance and removal of the toner image in the preceding stage and carrier deposition are generated as explained previously.
On the other hand, the multicolor electrostatic recording apparatus disclosed in the official gazette of the Japanese Patent Laid-Open No. HEI 2-77767 is difficult, when the non-contact development system utilizing the double-element developer is applied, to establish compatibility of sufficient reproduction of image density and prevention of image fault such as mixing of colors, mixed migration and carrier deposition, only with the setting of the developing bias waveform where a half value (1/2) of the maximum voltage in one period of the waveform is different from average voltage value.
It is because the toner and carrier are charged in the inverse polarities and thereby these elements receive an electrostatic force to move in the opposite directions with each other under the equal electric field, resulting in the requirement that the developing bias voltage must be set considering the movements of both toner and carrier depending on the effect of the electric field.
Meanwhile, the multicolor electrostatic recording apparatus of the type explained previously has such a technical problem that when the amount of charges of the carrier increases even if the maximum electric field working for attracting the toner image of the preceding stage on the latent image carrier to the development roll is set to 2.3 V/m or less, electrostatic vibration of carrier due to the effect of the vibration voltage increases, causing the developer layer to be placed in contact with the surface of the latent image carrier, followed by disturbance of toner image in the preceding stage, mixed migration and carrier deposition.
Moreover, the image forming apparatus disclosed in the official gazette of the Japanese Patent Laid-Open No. HEI 3-206473 has a technical problem that since tone reproducibility changes depending on the duty ratio and peak value of the developing bias voltage, the tone control method is different for each developer unit and thereby the tone reproducibility control under the change in amount of charge of developer and environmental change may be complicated. Particularly, when a large number of developer units are used, for example, when a full-color image is to be formed with the developer units for four colors of black, yellow, magenta and cyan, the above-mentioned image forming apparatus has a technical problem that the tone reproducibility control is extremely complicated.
Furthermore, in the image forming apparatus of the type explained above, since the development poles are arranged at the position other than the area where the development roll and the latent image carrier are provided closest with each other, a magnetic brush is intensively constrained on the development roll with the effect of the magnetic field in the horizontal direction at the area nearest the latent image carrier. Therefore, this image forming apparatus has a technical problem that it is difficult to obtain sufficient development density because the toner is developed from the upper most layer of the magnetic brush.
Meanwhile, the above image forming apparatus has another technical problem that it is difficult to establish compatibility against mixing of color on the toner image in the preceding stage on the latent image carrier and fogging on the background section because the vibration field intensity must be increased to obtain sufficient development density.